In the fixed bed coal gasification process, coal enters the gasifier at the top and lands on the previous charge. Steam and an oxidizing gas are admitted to the underside of the gasifier vessel. As the combustion being carried near the bottom of the gasifier consumes some of the coal and the rest of the coal is gasified, coal deposited at the top of the charge gradually moves down through the gasifier vessel passing through a series of treatment stages: initial heating; devolatilization and coking; gasification; and carbon oxidation. A large number of chemical reactions occur during these stages with a minimum temperature of about 1700.degree.-1900.degree. F being required. When air and steam are introduced to the combustion zone, the end product is producer gas; when oxygen and steam are employed, the end product is synthesis gas.
In the typical fixed bed coal gasifier the coal is introduced into the gasifier by means of a charging lock (lockhopper) arrangement and the ash is removed from the gasifier by means of a discharge lock. Each lock is provided with closure arrangements. These charging and discharge lock arrangements are troublesome in that the valves utilized to provide the necessary closure tend to leak as deposits of wet coal fines or ash accumulate on their surfaces. Removing and cleaning such valves incurs high maintenance costs and currently requires shutdown of the gasifier. Moreover, most valves in current use such as cone valves or ball valves are long in the flow direction which adds height and cost to the gasifier assembly.